<html maaa=a >
   <head>
      <style>
          body{
              background-color: #000;
          }
      body #container{
        width: 500px;
        height: 300px;
        display: flex;
        background-color: rgb(255, 255, 255)
      }
      body #container #myid{
        width: 200px;
        height: 100px;
        background-color: rgb(255, 0, 0)
      }
      body #container .c1{
        flex:1;
        background-color: rgb(0, 255, 0)
      }
      </style>
    </head>
    <body>
      <div id="container">
       <div id="myid"></div>
       <div class="c1"></div>
      </div>
    </body>
    </html>